1. Field of the Invention
The present invention relates to switching power converters and, more specifically, to reducing EMI (Electro-Magnetic Interference) caused by ring oscillation in switching power converters.
2. Description of the Related Arts
Compact and efficient switching power supplies are enjoying increasing popularity because they offer both compactness and high operating efficiency. Low efficiency linear power supplies use highly dissipative series pass elements to achieve output regulation. Switching power supplies achieve high efficiency by controlling ON and OFF power cycles of the power switch, delivering precise amounts of energy to the load while maintaining output regulation. The timing of the ON and OFF cycles determines the operating frequency of the switching power supply. The control of the ON and OFF cycles is typically achieved via well known modulation techniques, most notably pulse-width-modulation and/or pulse-frequency-modulation. Power transistors such as a bipolar transistor or a MOSFET device are employed as the switching device in switching power supplies, and are employed in one of a number of conventional topologies, such as flyback, buck, boost, buck-boost, etc.
It is desirable to operate the switching power converter at a high operating frequency in order to reduce the size of power conversion and filter components. These components include power transformers and inductors as well as output filter components such as capacitors and inductors. Depending on the topology of the switching power converter, output power, and other factors, the typical operating frequency of switching power supplies ranges from 10 KHz to 2 MHz.
However, one disadvantage resulting from the high operating frequency of switching power supplies is the generation of high frequency oscillation noise created by parasitic elements present in the switching power supply circuit. The frequency of the high frequency oscillation noise is typically many times higher than the operating frequency of the switching power supply. Furthermore, the EMI filter circuit of switching power converters is designed for optimal effectiveness at the operating frequency of the switching power converters, and is therefore ineffective at suppressing the high frequency oscillation noise caused by the parasitic elements. Therefore, additional circuitry is required to specifically suppress and absorb the high frequency oscillation noise.
Conventional techniques for suppressing the high frequency oscillation noise in switching power converters include the use of a special EMI filter (such as common mode inductor) or resistor and capacitor (RC) snubber circuits that are frequency matched to the high frequency oscillation noise. This technique has a number of disadvantages. First, they are designed to absorb the noise energy, and therefore are dissipative, negatively impacting the operational efficiency of the switching power supply. Second, the parasitic elements that cause the high frequency oscillation noise vary from component to component and thus from power supply to power supply. This, in combination with the component tolerance of the snubber circuit components, limits the effectiveness of the snubber circuit, especially in products that are in high volume manufacture. Third, snubber circuits or special EMI filters add to the cost and complexity of the switching power supplies.
The high frequency oscillation noise is a major cause of EMI and RFI (Radio Frequency Interference) noise. In the case of AC to DC power supplies, it is desirable to prevent noise from being transferred to the AC mains, as it negatively impacts the operation of other equipment connected to the AC mains. AC to DC switching power supplies often include an EMI filter circuit which is positioned between the AC mains and the EMI-generating switching circuit. The EMI filter is specifically designed to absorb the EMI energy produced by the switching power supply and preventing the EMI energy from being transferred to the AC mains. Once again, this has the same disadvantages as the snubber circuit or special EMI filter described above is dissipative, has limited effectiveness due to component tolerances, and adds to system cost and complexity.
Consider, for example, the conventional switching power supply illustrated in FIG. 1A, which shows a conventional AC to DC flyback switching power supply 100. The power converter 100 includes three principal sections, i.e., the front end, power stage, and secondary stage. The front end is directly connected to the AC voltage source 101, and includes EMI filter 102, bridge rectifier 103, and bulk capacitor 124. The output of the front end section is an unregulated DC input voltage 104. The EMI filter 102 is generically represented by two line-to-line capacitors 150, 152, and a common mode inductor 154, but designs can vary by power supply design.
The power stage is comprised of a power transformer 111, power switch 107, and switch controller 105. Power transformer 111 includes a primary winding 113, a secondary winding 114, and a parasitic capacitance 112. In addition to the switch element 162, power switch 107 comprises a parasitic switch capacitance 108. Switch controller 105 maintains output regulation via control of the ON and OFF states of power switch 107 via a control signal 106. Power supply controller 105 can employ any one of a number of well known modulation techniques, such as pulse-width-modulation (PWM) and pulse-frequency-modulation (PFM), to control the ON and OFF states and duty cycles of power switch 107.
The secondary stage is comprised of output rectifier 116 and output filter 117. The output filter 117 is generically represented by two output filter capacitors 156 158 and an output filter inductor 160, but designs can vary. The resulting regulated output voltage 118 is delivered to the load 119.
FIG. 1B illustrates the basic operational waveforms of the flyback switching power supply 100 of FIG. 1A. As explained above, switch controller 105 outputs a control signal 106 (in voltage form), which defines the ON and OFF states of power switch 107. Primary current 110 illustrates the current through power switch 107 and the primary winding 113. Referring to FIG. 1B in conjunction FIG. 1A, when control signal 106 is high and thus power switch 107 is in the ON state, primary current 110 ramps up. The rate of ramp up of primary current 110 is predominantly based on the DC input voltage 104 and the magnetizing inductance of primary winding 113. The voltage across power switch 107 is illustrated as switch voltage 109. When power switch 107 is in the ON state, switch voltage 109 is equal to 0 volt (not including voltage created by such factors as the drain-source resistance Rds-on of power switch 107). Furthermore, output rectifier 116 is reversed biased, and output current 115 is equal to 0 A. Thus, while power switch 108 is in the ON state, energy is stored in power transformer 111. When control signal is 0 volt (low) and the power switch 107 is switched to the OFF state, output rectifier 116 becomes forward biased and energy stored in power transformer 111 is delivered to the secondary side based on the turns ratio of the primary winding 113 and the secondary winding 114. As the energy stored in power transformer 111 is delivered to the secondary stage, secondary current 115 spikes up and then starts to ramp down. When all of the energy stored in power transformer 111 is delivered to the secondary stage, secondary current 115 becomes equal 0 A, which point is also referred to as the transformer reset point 126. If power switch 107 remains in the OFF state beyond the transformer reset point 126, the switch voltage 109 exhibits high frequency ringing, generally occurring during voltage ringing period 125.
High frequency noise, generally occurring during voltage ringing period 125, is caused by parasitic elements in the switching power supply circuit and is predominantly caused by the magnetizing inductance of primary winding 113, the parasitic capacitance 112 of primary winding 113, and switch capacitance 108. FIG. 1C is a simplified equivalent circuit of magnetizing inductance of primary winding 113, the parasitic capacitance 112 of primary winding 113, and switch capacitance 108. As shown in FIG. 1C, when power switch 107 is turned OFF, parasitic capacitance 108 and parasitic capacitance 112 are coupled in series to the magnetizing inductance of primary winding 113, and generates high frequency noise during voltage ringing period 125. The high frequency noise is generally many times higher in frequency than the operating frequency of switching power supply 100, and is a major cause of EMI/RFI noise. This requires special filtering circuitry to absorb the noise energy caused by these parasitic elements. EMI filter 102 is principally responsible for absorbing EMI noise energy in order to prevent EMI noise produced by the switching power supply 100 from being transferred to AC voltage source 101.